The crystallization of the PLLA block within the microphase-separated lamellar microdomain in a poly(styrene)-b-poly(L-lactide) (PS-PLLA) diblock copolymer was carried out from hard confinement (i.e., the crystallization temperature of PLLA (T-c,T-PLLA) < the glass transition temperature of PS (T-g,T-PS)) to soft confinement (i.e., T-c,T-PLLA greater than or equal to T-g,T-PS), where interesting morphological evolution was observed. At T-c,T-PLLA < T-g,T-PS, a typical confined morphology was obtained, while at T-c,T-PLLA greater than or equal to T-g,T-PS, a unique undulated morphology was seen for the first time. The amplitude and periodicity of the undulation instability are dependent upon the orientation of microphase-separated lamellae. The PLLA crystals under confinement formed perpendicular morphology having crystalline chains normal to the lamellae as evidenced by 2D small-angle X-ray scattering and wide-angle X-ray scattering. On the basis of morphological observations and crystallization kinetics studies, the undulation instability can be attributed to the change of crystallization mechanism from hard to soft confinement, which coincides with the transition from homogeneous to heterogeneous nucleation.